The use of wind turbines to harness wind energy in order to generate electrical power has a number of benefits, including “greenness,” i.e., wind turbines generally do not pollute the environment during normal operation, and the ability to provide electrical power to remote locations not having practical access to a wide-area power distribution network, among others. The most basic parts of a wind turbine are an electrical generator and a wind rotor (as distinguished from a generator rotor) that drives the generator as a result of a wind's effects on the wind rotor. As used herein, the term “wind rotor” denotes the assembly that comprises a blade hub and a plurality of blades (airfoils). Generally, the wind rotor converts wind energy into the rotational energy that drives the generator. Most early wind turbines included a gearbox connected between the wind rotor and generator so as to drive the generator at a different rotational speed than the rotational speed of the wind rotor.
Although gear-driven wind turbines are still being made and used, direct-drive wind turbines are becoming more prevalent largely due to advances in systems for controlling this type of wind turbine. As its name implies, direct-drive wind turbines do not include a gearbox, but rather have a direct mechanical coupling between the wind rotor and generator so that the wind drives the wind rotor and the rotor within the generator together as a unit. Direct-drive wind turbines are typically heavier than gear-driven wind turbines of comparable power output largely due to force transfer issues arising from directly coupling the wind rotor to the generator. Although direct-drive wind turbines are typically heavier than their gear-driven counterparts, direct-drive wind turbines have an important advantage in that their complexity is less than the complexity of their gear-driven counterparts. Direct-drive wind turbines simply have fewer moving parts. This lower complexity typically results in direct-drive wind turbines being more reliable than their gear-driven counterparts. Reliability is an important consideration for wind turbines, particularly wind-turbines used in remote locations that rely heavily on only one or a few wind turbines to provide the needed electrical power.
One important consideration in designing wind turbines of all types is to provide a robust structure while at the same time minimizing complexity, weight and amount of material needed to fabricate the wind turbines. Other important design considerations are maximization of accessibility to personnel for periodic inspection and/or maintenance and provision of a reliable and effective braking system for slowing, stopping and/or keeping stopped the wind rotor and generator periodically, e.g., to avoid damage due to overspeed, for maintenance and for other reasons.
A variety of conventional configurations exist for direct-drive wind turbines. Several of these configurations are described below for the purpose of illustrating conventional design approaches and shortcomings of these approaches in the context of the design considerations discussed immediately above.
World Intellectual Property Organization (WIPO) Publication No. WO 02/057624 to Wobben discloses a single-bearing, direct-drive, horizontal-axis wind turbine, which is indicated in FIG. 1 by the numeral 10. Generally, wind turbine 10 includes a wind rotor 12 and a generator 14 supported by a turret 16. Turret 16 is connected to a two-piece hollow spindle 18 consisting of parts 20, 22. Generator 14 includes a rotor assembly 24 and a stator assembly 26. Spindle 18 supports a plurality of radial support arms 28, which support stator assembly 26 of generator 14. Spindle 18 also supports a single bearing 30 that supports wind rotor 12 and rotor assembly 24. Wind rotor 12 is spaced from bearing 30 via an intermediate connecting shaft 32 that tapers inward toward the rotational axis of wind rotor 12 and rotor assembly 24 from bearing 30 to wind rotor, making access to the bearing difficult, if not impossible. In order for personnel to access bearing 30, wind rotor 12 and rotor assembly 24 would have to be removed. Thus, an inspection of bearing 30 that could otherwise be a relatively simple task, would require a crane, helicopter or other hoisting means and a great deal of effort. In addition, the active length of generator 14 is relatively small compared to its diameter. This is very efficient from an electrical standpoint, but inefficient structurally. This design requires relatively large and stiff radial support arms 28 to maintain the position of stator assembly 26. The design would be less expensive with a smaller diameter and longer active length, due to the decreased weight of radial support arms 28. Wobben is completely silent on any sort of frictional braking system for rotor assembly 24.
WIPO Publication No. WO 01/21956 to Lagerwey discloses another single-bearing, direct-drive, horizontal-axis wind turbine, which is indicated in FIG. 2 by the numeral 40. Wind turbine 40 comprises a wind rotor 42 and a generator 44 that includes a stator assembly 46 and rotor assembly 48 generally located radially outward from the stator assembly. A turret 50 supports a single-piece hollow spindle 52, which supports stator assembly 46. Spindle 52 also supports a single bearing 54 that supports wind rotor 42 and rotor assembly 48. Spindle 52 tapers to a smaller diameter from turret 50 to bearing 54. This configuration helps to carry the increased bending load in spindle 52 near turret 50. It also decreases the radial distance from spindle 52 to stator assembly 46, which decreases the weight and increases the stiffness of the support provided to the stator assembly. Rotor assembly 48 includes a rotor support 56 attached to bearing 54 and wind rotor 42. A shortcoming of this configuration relates to the stiffness of rotor support 56. In order to provide sufficiently stiff support, rotor support 56 would need to be relatively thick so as to keep the generator rotor precisely positioned relative to stator assembly 46. However, making rotor support 56 relatively thick is uneconomical. On the other hand, if rotor support 56 is too flexible, catastrophic rubbing between the rotor assembly 48 and stator assembly 46 will result.
FIG. 3 shows an alternative configuration for supporting the parts of generator 44 in FIG. 2. In this alternative configuration rotor assembly 48′ is generally located radially inward of stator assembly 46′. Stator assembly 46′ includes a stator support 60. Similar to rotor support 56 of FIG. 2, shortcomings of stator support 60 lie in its wall-type design. If stator support 60 is too thin, it will be unsuitable for precise control of stator assembly 46′ and catastrophic rubbing would likely result. On the other hand, if stator support 60 is thicker so as to provide adequate stiffness, the thickness results in economical inefficiency. Also, bearing 54′ is located axially forward of the rotor assembly 48′. This arrangement wastes axial space. In addition, this design of FIG. 3 uses air-cooled fins 62 in combination with direct cooling of the active portion of stator assembly 46′ using liquid cooling tubes (not shown). This is a relatively expensive and inefficient combination. Air cooling is passive and does not keep the temperature within set boundaries. Cooling tubes are inserted into holes in stator assembly 46′ and do not have sufficient direct contact with the active portion of the stator assembly needed for efficient heat transfer. Also, rotor assembly 48′ and wind rotor 42 are both connected into the outer race 64 of the bearing 54′. This requires outer race 64 to be drilled and likely threaded, which is a very expensive operation on a hardened bearing of this size. The seals of the bearing are not shown, but presumably the downwind seal is difficult to reach, since this seal would be nearly entirely enclosed by stator support 60. Only a small gap exists between rotor assembly 48′ and spindle 52′. Like Wobben, Lagerwey is completely silent on a frictional braking system for generator rotor 48′.
U.S. Pat. No. 6,452,287 to Looker discloses a ducted horizontal-axis, direct-drive wind turbine having a single-bearing. The Looker wind turbine has an integral wind rotor hub and generator rotor. The design has an impractical construction, however, for large wind turbines. The sections shown would be massive, expensive and difficult to lift for a large wind turbine. A more efficient structure is needed. In addition, no means is shown for practically connecting the bearing to the rotor and stator in such a way as to safely transmit the loads from the variations of the wind. Maintenance, moisture control and a braking system for the device are subjects clearly beyond the scope of the Looker disclosure.